The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to automotive substrates). In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous emulsion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. Early electrodeposition was conducted with the article to be coated serving as the anode. This was familiarly referred to as anodic electrodeposition. Currently, the article to be coated typically serves as the cathode in the electrical circuit with the counter-electrode being the anode. This is known as cathodic electrodeposition.
Resin compositions used in cathodic electrodeposition baths are also well known in the art. These resins are usually manufactured from polyepoxide resins which have been chain extended and adducted to include a nitrogen atom. The nitrogen is typically introduced through reaction with an amine compound Normally these resins are blended with a crosslinking agent and then neutralized with an acid and dispersed in water to form an emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to form the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and so forth.
The coated object is removed from the bath after a fixed period of time (normally about two or three minutes). The object is rinsed with deionized water and the coating is cured, typically in an oven at sufficient temperature to produce crosslinking.
The curing agents for anodic electrodepositable resins were usually aminoplasts or phenolplasts since these cure quite successfully. However, it was initially found that the aminoplasts were not completely satisfactory for use in cathodic electrodeposition. This is because aminoplasts cure best in an acidic environment. With anodic electrodeposition, this poses no problem since the anodically electrodeposited coating is acidic. However, the cathodically electrodeposited coating is basic and relatively high temperatures, that is, about 400.degree. F. (204.degree. C.) or higher must be used for complete curing of the cathodically electrodeposited coating.
Attempts have been made to overcome this problem by utilizing an acid-functional aminoplast as crosslinker with the hydroxyl containing amino epoxy resin (U.S. Pat. No. 4,066,525). However, this approach has not been found to be satisfactory because a high cure temperature of over 175.degree. C. is required. Other approaches include using quaternary onium salt-containing resins in combination with an aminoplast or a methylol-phenol ether (U.S. Pat. No. 3,937,679) disclosing 204.degree. C. cure tempearature. U.S. Pat. No. 4,501,833 also discloses quaternary onium salt containing resins in combination with high imino functional aminoplasts. While the '833 patent discloses relatively low cure temperature, we have found performance is not satisfactory because the coated film is rough and too thin.
Another approach is disclosed in U.S. Pat. No. 4,363,710 utilizing a resin with primary amino functionality and a melamine/formaldehyde crosslinker, catalyzed with a phenolic blocked phosphoric acid ester. However this system shows only very high temperature cure (180.degree. C. or above for 20 minutes).
There is a need for a cathodic electrodeposition process using aminoplasts which will give a good smooth coating and yet cure at low temperatures in a basic environment. We have found that an aminoplast system will cure at low temperatures (120.degree. to 150.degree. C.) in a basic environment (i.e. cathodic system) if catalyzed by metal catalysts. The metal catalysts are metal salts of both organic acid salts or inorganic acid salts such as Cu, Fe, Mn, Co, Pb, Bi, Zn and Sn octoate and napthanate. As stated above, this result is very surprising, as it was previously thought that aminoplast resins would only cure in an acid environment (anodic system) at these relatively low temperatures.
Metal catalysts are known in the art to catalyze certain coating compositions but metal catalysts are not known to cure aminoplasts. Prior art references teach the use of metal catalysts for the following: alkyd oxidative cure (U.S. Pat. No. 4,495,327); in an electrocoat system for transesterification (U.S. Pat. No. 4,352,842 and U.S. Pat. No. 4,644,036); and in electrocoat systems for transamidation (U.S. Pat. No. 4,477,530). There is nothing in the prior art to suggest their use to catalyze the reaction of aminoplast resins.
The novel resin of this invention is not restricted to cathodic electrodeposition. It also could be used in non-electrocoat applications such as spray applications, roller coating, dip applications, and so forth.